What topics can I ask about here?

Raspberry Pi Stack Exchange targets usage and development of hardware or software for Raspberry Pi.

Please make sure that your questions are intrinsically Raspberry Pi oriented. We have an unusual amount of "off-topic" questions because of the extensive overlap with our sibling sites at Unix & Linux, Stack Overflow, and Electrical Engineering. When in doubt, ask there first. Questions where the Raspberry Pi is not a significant factor will likely be closed.

If your question is about:

Raspberry Pi hardware, including GPIO and other related electronics.

Raspberry Pi peripherals.

Software specific to the Raspberry Pi.

Operating Systems built for the Raspberry Pi.

Materials published or provided by the Raspberry Pi Foundation.

… then you’re in the right place to ask your question!

But please note that the following is off topic

Raspberry Pi orders or delivery.

Asking for specific purchasing recommendations: This refers to questions such as, "What power supply should I buy?" (= off-topic). However, if you have a specific product and you want to know if it is suitable for a specific purpose ("Is this power supply okay for a Pi Zero W?"), that's fine. Asking what to look for in a kind of product is also fine ("What is the minimum amperage power supply I can use with the Pi 3?").

Asking for references to online material (use a search engine, and if that does not work, ask about the issue directly).